Recent interest in molecular beam epitaxy (MBE) stems from the recognition that MBE permits the fabrication of thin epitaxial layers with great uniformity over a large surface area, and with precision control of the layer thickness. Conventional techniques and apparatus for epitaxial growth of thin films of semiconductor materials by MBE techniques are known.
In U.S. Pat. No. 3,615,931 to J. R. Arthur, Jr. (1971), there is described an MBE technique in which growth of a thin film results from the simultaneous impingement of one or more molecular beams of the constituent elements onto a heated substrate. Furthermore, MBE techniques for making a group III(a)-V(a) p-type thin film by doping with magnesium have also become known. For example, one such technique is described in U.S. Pat. No. 3,839,084 to A. Y. Cho, et al. However, the MBE growth of such p-type layer is constrained by the low doping efficiency of Mg, more of which will be described hereinafter.
Notwithstanding the important advances in MBE, the growing of GaAS and Ga.sub.1-x Al.sub.x As by molecular beam epitaxy at both low substrate temperature and high growth rates, especially under high arsenic pressures, yields crystal of poor electrical and optical properties. These results have been reported, for example, by R. A. Stall, et al, Electron. Letter 16 (5), pp. 171-2, (1980), and by T. A. Murotani, et al, Journal of Crystal Growth, 45, pp. 302-08 (1978).
MBE growth is also subjected to many parametric constraints. For instance, the minimum substrate temperature to form good to high quality GaAs is about 550.degree. C. as reported by A. Y. Cho, et al, Journal of Applied Physics, 43 (12), pp. 5118-5123 (1972). For Ga.sub.1-x Al.sub.x As with x&gt;0.1, the substrate must be maintained at 650.degree. to 700.degree. C. MBE growth rates are limited to about one micron-per-hour, with the best V-III flux ratios at about two-to-one.
In the fabrication of AlGaAs layers, it is recognized as important to reduce as much as possible the amount of deleterious contaminants. For instance, in U.S. Pat. No. 3,974,002 to H. C. Casey, Jr., et al, it is stated that deleterious contaminants, e.g., H.sub.2 O, CO, O.sub.2, and hydrocarbons are reduced as much as possible by utilizing pyrolytic boron nitride rather than graphite, effusion cells. Also described is a technique using relatively uncollimated beams so that a portion of the beams deposits continuously on the interior walls of the vacuum chamber fresh layers which getter the deleterious contaminants.
Ga.sub.2 O has become known as a deleterious contaminant. For instance, in an article entitled, "Vapor Pressure of Gallium, Stability of Gallium Suboxide Vapor, and Equilibria of Some Reactions Producing Gallium Suboxide Vapor", by C. N. Cochran et al, Journal of Electrochemical Society, 109, No. 2, February 1962, pp. 144-148, the authors showed in essence that volatile Ga.sub.2 O is present and requires consideration in chemical reactions involving gallium.
As referred to above, one of the difficulties encountered in MBE growth is the low doping efficiency of magnesium as described by A. Y. Cho, et al, Journal of Applied Physics 43 (12), pp. 5118-5123 (1972). More specifically, the sticking coefficient (doping efficiency) of magnesium is described by A. Y. Cho, et al, in U.S. Pat. No. 3,839,084 as a non-linear, monotonically increasing function of the amount of aluminum in the MBE growth of Mg doped p-type thin film compound of Al.sub.x Ga.sub.1-x As. According to the Cho, et al, patent, the sticking coefficient of Mg can be varied by controlling the intensity of the Al beam (i.e., the Al arrival rate). As a result, for a given carrier concentration in the layer grown, a lower Mg beam flux may be used by increasing the amount of Al in the layer, i.e., by increasing the intensity of the Al beam.